Project Summary: This application is to continue on-going studies of the normal and pathological functions of the death effector domain (DED)-containing phosphoprotein PEA-15. The hypothesis is that PEA-15 regulates the decision of T lymphocytes and some cancer cells to proliferate or to die. This hypothesis is based on the following observations made during the current funding period: PEA-15 is an ERK-binding protein that restricts ERK1/2 to the cytoplasm and thereby prevents ERK activation of nuclear targets. The ERK substrate RSK-2 is also a PEA-15 binding partner and PEA-15 enhances ERK activation of RSK-2. Indeed PEA-15 knock-out thymocytes do not activate RSK-2 in response to ERK activation. RSK-2 can promote cell survival and proliferation. In contrast, PEA-15 has also been reported to inhibit death receptor activated apoptosis by binding to FADD and preventing recruitment and activation of caspases. Phosphorylation of PEA-15 was found to control which of these pathways is affected. In aim 1) the molecular mechanism by which PEA-15 regulates ERK MAP kinase signaling and FADD-mediated apoptosis will be further characterized using both cell culture and knock-out derived cells. In PEA-15 knock-out mice T lymphocytes undergo uncontrolled expansion in response to T-cell receptor ligation. In aim 2) the immune phenotype of PEA-15 knock-out mice and the mechanism responsible for the increased number of T-Cells will be determined in extracted lymphocytes and in immune challenged mice. Abnormal expression of PEA-15 promotes tumor formation and may effect resistance to apoptosis-directed chemotherapies. In epithelial cells, PEA-15 in combination with activated H-Ras promotes greater tumor growth than H-Ras alone. In addition, cells expressing PEA-15 show significantly reduced migration. In aim 3) the effects of PEA-15 on Ras transformed epithelial cell growth and metastasis will be investigated both in vitro and in vivo using mouse epithelial cells. Relevance to public health: Control of cell proliferation and apoptosis is finely tuned in T cells during immune challenge and is set in cancer cells for optimum proliferation with minimal apoptosis. PEA-15 is a protein that sits at a nexus of control for these two events and may therefore tip the scales one way or the other. At the conclusion of these studies the molecular mechanisms of PEA-15 normal function in T cell immune response and abnormal function in cancer will be better understood. This will determine if PEA-15 is a suitable target for the development of new cancer therapies and if it can be used as a marker for cancer progression.